Question:

1. List the classes, frequencies and find the class‑midpoints
\[
\begin{array}{c|c|c}
\text{Class (ppm)} & f_i & x_i \text{ (midpoint)}\\ \hline
0.00-0.04 & 4 & \frac{0.00+0.04}{2}=0.02\\
0.04-0.08 & 9 & \frac{0.04+0.08}{2}=0.06\\
0.08-0.12 & 9 & \frac{0.08+0.12}{2}=0.10\\
0.12-0.16 & 2 & \frac{0.12+0.16}{2}=0.14\\
0.16-0.20 & 4 & \frac{0.16+0.20}{2}=0.18\\
0.20-0.24 & 2 & \frac{0.20+0.24}{2}=0.22\\
\end{array}
\]

2. Calculate \(f_i x_i\) for each class
\[
\begin{array}{c|c|c|c}
\text{Class} & f_i & x_i & f_i x_i\\ \hline
0.00-0.04 & 4 & 0.02 & 4\times0.02 = 0.08\\
0.04-0.08 & 9 & 0.06 & 9\times0.06 = 0.54\\
0.08-0.12 & 9 & 0.10 & 9\times0.10 = 0.90\\
0.12-0.16 & 2 & 0.14 & 2\times0.14 = 0.28\\
0.16-0.20 & 4 & 0.18 & 4\times0.18 = 0.72\\
0.20-0.24 & 2 & 0.22 & 2\times0.22 = 0.44\\
\end{array}
\]

3. Add the frequencies and the products
\[
\sum f_i = 4+9+9+2+4+2 = 30
\]
\[
\sum f_i x_i = 0.08+0.54+0.90+0.28+0.72+0.44 = 2.96
\]

4. Compute the mean
\[
\bar{x}=\frac{\sum f_i x_i}{\sum f_i}=\frac{2.96}{30}=0.09866\text{ ppm}\approx 0.10\text{ ppm (to two decimal places)}
\]

5. Answer: The mean concentration of SO₂ in the air is approximately 0.10 ppm.